The present invention relates to a scanning photon microscope that enables observation of characteristic distribution of a specimen by sweeping a chopped light spot across the specimen such as semiconductors, by detecting an ac photovoltage or photocurrent generated in the specimen, and by displaying the two-dimensional distribution of the photovoltage in the form of black and white images. More specifically, this invention relates to an apparatus that permits multilateral analysis of the characteristics of the specimen by simultaneously displaying on a CRT (cathode ray tube) screen the amplitude and the phase of the specimen's ac photovoltage or photocurrent.
In performing non-destructive inspection on semiconductor wafers for their electrical characteristics, scanning photon microscopes utilizing the photoelectric effect have been used.
In such microscopes, the characteristic distribution of semiconductor wafers is examined in a way as described in say Japanese Patent Application Laid-Open No. 155543/1981. That is, an ac photovoltage generated in the semiconductor wafer specimen by the chopped photon beam sweeping across the specimen is detected and the voltage distribution is displayed on the CRT screen as a black and white image.
One example of the operating principle of the scanning photon microscope that precedes this invention and uses the aforementioned conventional technique is shown in FIG. 2.
In FIG. 2, a photon beam from the light source 1 such as laser is modulated in intensity by optical modulator 2 according to ac signals from oscillator 7. The modulated beam is passed through a photon beam deflector 3 into a lens 4 which focuses the beam onto a specimen 5. As a result, the specimen 5 such as semiconductor wafers with a junction produces within it an ac photocurrent by the photoelectric effect and induces an ac photovoltage which is a product of the photocurrent and the junction impedance.
The induced ac photocurrent or ac photovoltage is amplified by an ac amplifier 6 and supplied to synchronizing rectifiers 11 and 12. The synchronizing rectifier 11 performs synchronism rectification on the output of the amplifier 6 according to a reference signal from a phase shifter 9 that shifts the phase of the ac signal from the oscillator 7.
The phase shifter 10 produces another reference signal which is delayed 90.degree. from the output signal of the phase shifter 9. On the basis of this reference signal, the synchronizing rectifier 12 performs synchronism rectification on the output of the amplifier 6. The outputs of the synchronizing rectifiers 11 and 12 are supplied to integrators 13 and 14 respectively which calculate the averages of the synchronization rectification results.
The above operation is further explained referring to the waveforms shown in FIGS. 3A to 3F. A waveform denoted 30 in FIG. 3A--which is the ac output signal from the oscillator 7--is phase-shifted by the phase shifter 9 to form a reference signal indicated by a waveform 31 of FIG. 3B. The reference signal 31 is then supplied to the phase shifter 10 that produces another reference signal 32 of FIG. 3C which is lagging the first reference signal by 90.degree..
Synchronizing rectification performed on the output of the amplifier 6, indicated by waveform 33 of FIG. 3D, according to the reference signals--waveforms 31 and 32--produces waveforms 34 and 35 of FIGS. 3E and 3F. Waveforms 36 and 37 represent the average values of the synchronizing rectification results, obtained by supplying these waveforms 34 and 35 to the integrators 13 and 14.
Now, turning to FIG. 2, a calculator 22 determines the amplitude of the output signal from the amplifier 6 by processing the outputs from the integrators 13 and 14. In more detail, the calculator performs the operation of ##EQU1## where A is an amplitude, X and Y are the outputs of the integrators 13 and 14 respectively. The calculator 22 feeds its operation result to a CRT driver 23 and further to a monochromatic CRT 24 where the amplitude of the signal is converted into the varying degrees of light spot brightness on the phosphor screen of the CRT 24.
The scanning movement of the light spot on the specimen 5 is controlled by the photon beam deflector 3 which is driven by a scan control circuit 8. The light spot on the phosphor screen of the CRT 24 is controlled, according to the signals from the scan control circuit 8, by a CRT deflector driver 19 and a deflecting coil 20 in synchronism with the sweeping timing of the light spot on the specimen 5.
As a result, the ac photovoltage distribution induced in the specimen 5 is displayed as a black and white image on the phosphor screen of the CRT 24 with the photovoltage magnitude reflecting electrical characteristics of the specimen, thus making it possible to estimate the electrical characteristics of the specimen. A circuitry made up of the ac amplifier 6, phase shifters 9 and 10, synchronizing rectifiers 11 and 12, and integrators 13 and 14 forms what is generally called a lock-in amplifier.
However, as the black and white image thus obtained has entirely lost information concerning the phase of the ac photovoltage or photocurrent produced in the specimen 5, it is difficult to identify the polarity of the junction in the specimen or estimate the condition of the depletion layer. One of the existing methods available to overcome this drawback is to eliminate, in FIG. 2, the phase shifter 10, synchronizing rectifier 12, integrator 14 and calculator 22. With this method, however, the brightness of the light spot on the phosphor screen of the CRT 24 is determined by both the amplitude and phase of the ac photovoltage or ac photocurrent generated in the specimen 5. This means it is difficult to distinguish between the amplitude and the phase from the displayed image.
In other words, the conventional technique, because of its inability to tell the amplitude from the phase of the ac photovoltage or ac photocurrent induced in the specimen 5, cannot offer detailed analysis on the characteristics of the specimen.